1. Field of the Invention
This invention is in the field of printing, more specifically in the field of making flexographic printing plates with digital quality flat top dots using liquid photopolymers, and still more specifically in the field of using solvent-based inks to print the flexographic plate negatives.
2. Description of the Related Art
A prior art method of preparation of a simple flexographic (“flexo”) printing plate from cast liquid photopolymer currently involves the steps of (a) printing a black negative image on a white substrate; (b) photographing the negative image; (c) developing the film negative; (d) positioning the film negative on the bottom glass of a UV exposure unit; (e) positioning liquid dams and taping the negative in place with masking tape as necessary; (f) placing a thin plastic vacuum sheet (cover film) over the negative; (g) casting liquid photopolymer over the film to the proper depth; (h) applying a vacuum to the laminate thus formed (negative, vacuum sheet, and photopolymer); (i) exposing this laminate to UV (or other appropriate actinic) radiation through the negative for an amount of time sufficient to create a cross-linked polymerized image in the photopolymer; (j) removing the laminate from the unit and separating the vacuum sheet and negative from the cross-linked photopolymer; and (k) removing the unexposed material to develop a relief image.
The vacuum step (h) is important. If there are any pockets of air between the negative and the adjacent surface of the photopolymer, the UV light will be refracted by the interfaces between the film, the air, and the photopolymer and the final image after exposure will be distorted. If the pockets of air are large enough, they can even lead to mechanical failure of the plate by creating thin spots in the photopolymer. Moreover, ambient oxygen in any air pockets in contact with surface of the photopolymer inhibits full curing of the photopolymer all the way to the photopolymer surface. (This is thought to be because oxygen in the air reacts with gases formed within the photopolymer layer during UV curing, the products of which slow the curing rate.) Provided the negative and vacuum sheet are skillfully placed, the vacuum system in the special exposure unit will remove a great majority (but not all) such pockets of air. The tops of the half tone dots produced with distortion due to the presence of air are rounded; the printed images of such dots oftentimes appear as tiny squiggles where the dots should be. Even with perfect alignment and application of vacuum, single halftone or pixel dots produced this way suffer from being separated from the photopolymer by the additional thickness of a cover sheet. They do not have steep shoulders and cause dot gain when printed.